1. Field of the Invention
The present invention relates to a cathode of a lithium secondary battery and to a lithium secondary battery having the cathode.
2. Description of the Related Art
The cathode active material layer of a lithium secondary battery (may be referred to as a “lithium ion secondary battery”) is widely known to be formed through kneading a kneaded product of a lithium composite oxide (lithium transition metal oxide) powder with additives such as a binder and a conducting agent and molding the kneaded product. For example, Japanese Patent Application Laid-Open (kokai) No. Hei 5-226004 discloses such a technique. Hereinafter, such a configuration is referred to as a “powder-dispersion type cathode.”
Since the powder-dispersion type cathode contains a large amount (e.g., about 10 wt. %) of a binder, which is not a capacity-enhancing ingredient, the relative amount of the lithium composite oxide serving as a cathode active material contained in the cathode is small. Therefore, the capacity and charge-discharge efficiency of such a powder-dispersion type cathode are not satisfactory and to be further improved.
To overcome this drawback, efforts have been made to improve the capacity and charge-discharge efficiency through forming the cathode or the cathode active material layer from a sintered lithium composite oxide sheet. For example, Japanese Patent Application Laid-Open (kokai) Nos. Hei 8-180904 and 2001-143687 disclose such a technique. According to this technique, the cathode or the cathode active material layer contains no binder. Therefore, the lithium composite oxide filling density increases, conceivably leading to high capacity and excellent charge-discharge efficiency.
In the case where the cathode or the cathode active material layer is formed from a sintered lithium composite oxide sheet, when the sintered sheet has a low lithium composite oxide filling ratio (i.e., high voidage), the effect of enhancing the performance of the aforementioned powder-dispersion type cathode; in particular, capacity-enhancing effect, is unsatisfactory. In fact, cathodes formed from a sintered lithium composite oxide sheet disclosed in Japanese Patent Application Laid-Open (kokai) Nos. Hei 8-180904 and 2001-143687 have a low filling ratio (a voidage of 15 to 60%), which is unsatisfactory in terms of capacity.
Meanwhile, when the sintered sheets has an excessively high lithium composite oxide filling ratio, high capacity is attained, but the cyclic characteristic (i.e., capacity retention performance after repetition of charge-discharge cycles) is known to be problematically impaired. The impairment in cyclic characteristic is caused also when the sintered sheet has a thickness of about 10 μm. Particularly when the sintered sheet has a thickness as large as 30 μm or more, the impairment is considerably severe.
In order to elucidate the cause for the impairment, sintered lithium composite oxide sheets tested in an experiment example where the cyclic characteristic had been impaired were previously observed under an electron microscope. Through observation, cracks were found to be generated in the sintered sheets. The cracks were generated at the grain boundary, where the boundary between regions which are adjacent to each other and have different crystal orientations (hereinafter, the cracks are referred to as “grain boundary cracks”). Furthermore, the sintered lithium composite oxide sheets tested in the experiment example were observed under an electron microscope at the interface between the conductive bonding layer (disposed between the cathode collector and the sintered sheet) and the sintered sheet. Through the observation, separation (interspace formation) was found to occur at the interface (hereinafter referred to as “bonding interface separation”).
The grain boundary cracking is thought to be caused by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles, wherein the crystal lattice expansion/contraction includes the expansion/contraction of volume and the expansion/contraction not involving change in volume. Similarly, the bonding interface separation is thought to be caused by the tensile or shear stress generated between the sheets and the conductive bonding layer associated with the morphological change of the sintered lithium composite oxide sheet by crystal lattice expansion/contraction. Thus, such grain boundary cracking or bonding interface separation, which results in generation of an electrically isolated portion (i.e., a portion which does not contribute to capacity) through breakage of an electrical conduction path in the sintered lithium composite oxide sheet, to thereby reduce capacity, is thought to be a possible cause for deterioration in cycle characteristic.